The present disclosure relates to a methodology for improving the print quality of line-art corners and other fine details as found in both font and image data. Due to the nature of inks, paper and printing, these types of problems are well known in the printing arts.
In the printing arts this problem was typically overcome by manual image customization by hand of any troublesome detail areas. This was particularly the case with fonts or reusable type. For example, ink traps would be added to those areas in a font character where too much ink would otherwise collect and cause smearing. Similarly, detail areas would be sharpened to insure the desired print result.
This approach of compensating to get a desired result such as sharpening has followed-on from the print arts into the digital imaging arts. As an example, Digital Typography: An Introduction to Type and Composition for Computer System Design, by Richard Rubinstein, discusses the desirability of compensating for electrostatic effects which result in toner not being placed on the paper exactly as the bit image specifies. Compensation is depicted there as adding to the image bit map to sharpen convex (outside) corners which would otherwise get rounded over. An alternative compensation is also depicted for handling situations involving concave (inside) corners by removing black printing pixels from the corner region of a shape to make an ink trap. In FIG. 21 there is depicted the bit image data and thereby the desired print image. In FIG. 22 is depicted the rounded-over result when printed on a given printer. FIG. 23 shows the bit image compensation which when printed will yield the desired result as depicted in FIG. 21 above. Note the ears 2000 added to the outside corners and the ink trap 2010 added to address the inside corner.
In a digital image path for printing digital text and line art it is often desirable to perform anti-aliasing to better represent the contours of these image structures. Anti-aliasing refers to the use of a gray-value for a pixel to represent partial area coverage of that pixel by the edge of text or line art. Anti-aliased image structures, therefore possess gray edges. The presence of gray edges presents difficulties to image processing methods that modify text line art.
The following appear to be relevant to manipulation and enhancement of the edges of image shapes. They are incorporated by reference in their entirety for their teaching, particularly of template matching, and of bit map or signal substitution.
In U.S. Pat. No. 4,847,641 to Tung, print enhancement circuitry to enhance the printed image produced by a laser beam printer is interposed between the character generator circuits and the laser drive circuits to modify the laser drive signals provided by the character generator circuits. Bit data representing successive lines of the bit map for a desired image are stored in a first-in first-out (FIFO) buffer. The bit pattern sample window having a central cell (bit) and a selected (arbitrary) number of neighboring bits is compared to a number of matching bit patterns or templates, each of which is associated with an error element or cell. When a logic matching network detects a match, a modification signal associated with a unique compensation cell (bit) is generated. The sample window central bit is then replaced (modified) with the unique compensation bit required by the matching template. In this manner, all bits in a desired bit map, or set of bit maps, are examined and their corresponding laser drive signals modified to compensate for the errors associated with the matched templates in a piece-wise manner.
In U.S. Pat. No. 5,383,036 to Mailloux et al., a method for enhancing the contour fidelity of printed images of two or more colors is described, which includes obtaining a digital representation of the color image and finding color separations of each color. Each color separation is enhanced by a single set of inverse symmetrical templates, the set including templates in which the second template is always the inverse of the first, and the third and fourth templates are 180 degree rotations of the first two. The resulting smoothed color separations are recombined into an enhanced image without separation error.
In U.S. Pat. No. 4,437,122 to Walsh et al., presents an invention which enhances the resolution and quality of characters of a system receiving the information initially in the form of video display pixels and providing hard copy output. This is accomplished by storing at least three successive lines of video data in successive, parallel connected shift registers, applying the outputs of the shift registers to a decoder, and generating driving signals for the printer head. The decoder compares the pixels on the same line as well as in preceeding and succeeding lines that surround each specific input pixel to generate the printer head driving signals according to whether straight or curved line segments are to be formed. In effect, the printer increases the density of the information elements and simultaneously provides rounding off of character edges and smoothing of diagonals.
Therefore, there exists a need for a technique which will solve the problem of corner rounding as an automated, non-manual processing operation in an anti-aliasing image path. Thus, it would be desirable to solve this and other deficiencies and disadvantages with an automated processing methodology.
Disclosed in embodiments herein is a method of corner sharpening in the display of a bitmapped digital image. In one embodiment the corner sharpening method comprises identifying pixel locations about corner structure in a digital image and substituting pixel values in the identified pixel locations to achieve a clustering of the substituted pixel values about the corner structure; thus producing a corner-enhanced digital image Filtering and sampling according to an anti-alias operation is applied to said corner-enhanced image to produce an anti-aliased corner enhanced image.
In another embodiment a corner sharpening method comprises identifying pixel locations about corner structure in a digital image. Pixel values are substituted in the identified pixel locations to achieve a clustering of the substituted pixel values about the corner structure, producing a corner-enhanced digital image. Filtering and sampling according to an anti-alias operation is performed to said corner-enhanced image to produce an anti-aliased corner enhanced image. The sampling produces image data samples at a lower resolution, with the rendering of the lower resolution image data samples provided as suitable for driving a laser printer.